Equalization technology is commonly classified into two classes: linear equalization and nonlinear equalization. A linear equalizer is relatively simple, and can eliminate the effect of a channel effectively when a channel fading is not serious. A Zero Forcing (ZF) algorithm and a Minimum Mean Square Error (MMSE) algorithm are commonly used algorithms. When the multipath fading of a wireless channel is serious, a very deep zero point occurs in the frequency domain response of the channel. In order to compensate the amplitude fading near the zero point, the linear equalizer has to amplify the frequency spectrum of a corresponding frequency band. Thus, the noise of the corresponding frequency band is enhanced (especially for a ZF equalizer), and the Signal Noise Ratio of the system is reduced. Meanwhile, a nonlinear equalizer will have a better effect in such severe channel. A Decision Feedback Equalizer (DFE) is one of commonly used nonlinear equalizers.
Multi-carrier Orthogonal Frequency Division Multiplexing (OFDM) is a modulating manner for a frequency selective channel. K subcarriers with equal interval are employed, each subcarrier is modulated individually, and the symbol period is K times of that of a single-carrier system having a same rate. Multipath interference can be suppressed effectively. Moreover, the OFDM system inserts a guard space among respective symbols so as to eliminate inter-symbol interference. Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) can be used to achieve modulating and demodulating of an OFDM signal. Since a part of sub channels will also encounter deep fading and result in a high code error rate, which will result in a code error platform for the OFDM system, a strong error correction coding is commonly employed to correct the error. However, a long channel error correction coding will result in a long time delay of processing of the system. Therefore, a diversity of frequency domain is employed to improve the channel. That is, same information symbols are transmitted over different subcarriers. Diversity gain can be obtained when fading of respective subcarriers is independent, thereby avoiding the high code error rate of data over the subcarriers due to the deep fading.
A linear equalizer using a non-ZF algorithm such as a MMSE equalizer may reduce the amplification of the noise for the zero point in the frequency domain. However, a problem occurs that the equalizer remains the inter-symbol interference. The remained inter-symbol interference would significantly reduce the performance of the modulating system, resulting in the platform of the code error rate. The nonlinear equalizer DFE has a relatively high complexity of processing when the data rate is high. A problem regarding stability and decision code error propagation also occurs. Accordingly, both the linear and nonlinear equalizers have some defects. Especially, when a channel is severe, the time delay of the channel is long, resulting in a large inter-symbol interference and more zero points in the frequency domain. In such case, both the MMSE and DFE equalizers have bad performance.
Diversity technology of OFDM can improve the channel response, which is different from the channel equalization. Both the linear and nonlinear equalization is to accommodate severe channel conditions. However, a severe inter-symbol interference of the channel has occurred at this time, it is too late to make correction only using an equalizer. On the other hand, the OFDM has a complicated hardware structure and high energy consumption. As a signal is divided into a plurality of subcarrier signals, a problem occurs that the amplitudes of the subcarrier signals superimpose when symbols of all subcarrier signals are same, which is referred to as a Peak to Average Power Ratio (PAPR) problem. The PAPR problem causes the energy consumption for transmission to increase several dBs, and results in high requirement for Automatic Gain Control (AGC) and linearity of a high frequency component of a circuit. Therefore, the complexity, energy consumption, and cost of the OFDM are all high.